The intention is all "solo money objects" (objects that only one person can use at a time) give the same amount of money in the long run. The tasks last for different amount of times and so the payouts are different. I don't think they (EA/Maxis) managed to balance it out perfectly, though. It generally believed that chalkboards give the most money (if you are paying attention and click to sell as soon as you are able)
Group money objects (pizza, core business unit, tomb hunters of the lost llama, and the roadie band object) are all meant to give more money than solo objects since they require teamwork. The Maze (Tomb hunters of the lost lllama) object is not currently working, though. Code (Core Business Unit) is but people don't seem to play it very often. It is generally felt that the payout on it is "meh".